A mechanical tensioner is used to automatically control the tension of a belt of a front end accessory drive for automotive engine applications. Such a tensioner has a pivot-arm that rotates about a pivot secured to a base and uses a sleeve-type bushing on the pivot to provide a bearing surface for the rotating pivot-arm. A torsion spring is often used with one end connected to the pivot-arm and the other end interconnected through the base to bias the position of the pivot-arm and position an attached pulley against a serpentine belt. The spring is also used to generate a spring force operative with a damping means that generates a normal force component to a friction sliding surface to inhibit or dampen oscillatory movements of the pivot-arm.
Since the serpentine belt must be routed to all accessories, it has generally become longer than its predecessors. To operate properly, the belt is installed with a pre-determined tension. As it operates, it stretches slightly over its length. This results in a decrease in belt tension, which may cause the belt to slip. Consequently, a belt tensioner is used to maintain the proper belt tension as the belt stretches during use.
As a belt tensioner operates, the running belt may excite oscillations in the tensioner spring. These oscillations are undesirable, as they cause premature wear of the belt and tensioner. Therefore, a damping mechanism is added to the tensioner to damp operational oscillations.
Various damping mechanisms have been developed. They include viscous fluid dampers, mechanisms based on frictional surfaces sliding or interaction with each other, and dampers using a series of interacting springs. For the most part these damping mechanisms operate in a single direction by resisting a movement of a belt in one direction. This generally resulted in undamped vibrations existing in a belt during operation as the tensioner arm oscillated between loaded and unloaded positions.
The prior art systems rely on a tensioner set up to be compliant in order to follow the motion of the belt. Usually the tensioner is set up with a low damping rate to facilitate this compliance. As a result the prior art systems operated in an unsatisfactory manner during load changes. The accessory drive operated normally when the engine was running at a steady RPM. The tensioner bearing against the belt would maintain a tension in the span. Generally, the tensioner is “downstream” of the crankshaft in a belt movement direction. Damping was set so that the tensioner would damp most of the vibrations in the running belt.
The problems arise when the engine speed is rapidly changed, in the range of 5000 to 10000 RPM/sec. In this case, the accessories such as the alternator continue to drive the belt after a speed reduction due to rotational inertia. This causes the belt on the “downstream” side of the crankshaft to tighten, loading the tensioner. If the damping rate in the tensioner is too low the tensioner will be unable to resist the increase in belt tension and the arm will move in a direction away from the belt. As a result, the tensioner is not maintaining sufficient tension in the belt. This will allow the belt to slip on the crankshaft pulley, since the belt is now being driven toward the crankshaft, causing squeeking noises. Some prior art systems rely on a means of locking the tensioner arm in the loading direction to prevent the decrease in belt tension. However, locking the tensioner prevents the tensioner from performing its corollary function of damping vibrations in the belt.
Many of the prior art systems depend upon a locking tensioner or upon a particular mechanical arrangement to address the problem of high rate of change of engine speed. Neither system solves the dual problems of preventing squeal during speed changes while continuing to damp belt vibrations. Further, the prior art systems, can be complex and expensive, requiring complex mechanical devices to control the movement of a tensioner arm. The prior art systems are relatively large requiring room on the engine surface.
As a result asymmetric tensioners where developed which vary the damping force depending upon the loading direction of the tensioner. This allowed a high damping rate to be applied in the loading direction while a significantly reduced damping rate was applied in the unloading direction. These tensioners comprised a torsion spring that engage a damping mechanism at two contact points thereby creating a torsional couple which causes the damping mechanism to exert a normal force on the damping mechanism frictional surface. The torsion spring operates to apply the couple to the damping mechanism in the winding direction.
Representative of the art is U.S. Pat. No. 6,609,988 which discloses an asymmetric damping tensioner system for belt drives on an engine. A belt is connected between a driver pulley on a crankshaft and any number of driven pulleys. Each driven pulley is connected to an accessory such as an alternator, power steering pump, compressor or the like. The tensioner is placed anywhere before the first component of significant effective inertia, in the belt movement direction. A biasing member in the tensioner is used to maintain a tension in the belt. The tensioner further comprises a damping mechanism to damp belt vibrations caused by the operation of the engine. Tensioner damping friction is unequal or asymmetric, depending upon the direction of movement of the tensioner arm. During acceleration the damping friction of the tensioner in the unloading direction is significantly lower than the damping friction in the opposite, or loading direction, as is the case during deceleration. Lower damping friction during acceleration allows the tensioner arm to quickly adjust to the increase in belt length caused by acceleration. Higher damping friction during deceleration prevents the tensioner arm from being moved too far in the loading direction thereby causing slipping and noise. Asymmetric damping also significantly diminishes overall vibration in the belt during all phases of operation.
What is needed is a tensioner having a torsion spring having a second end bearing upon the damping member so that upon loading of the torsion spring in the unwinding direction the damping member is compressed between the second end of the torsion spring and the base thereby causing a normal force to be imparted upon the pivot arm by said damping member. The present invention meets this need.